The present invention relates to a surface acoustic wave (SAW) device that is operated at a frequency band of 10 GHz, that can be produced with a mass-producible line width of 0.5 xcexcm or more in its interdigital transducers (IDTs), and that has excellent operational performance.
SAW devices rely on a SAW that propagates with its energy concentrated at the surface of a solid. Being small, easy to produce, and stable in temperature properties, they are used in applications such as filters for TV receivers. Generally, SAW devices are provided with IDTs on a piezoelectric body. A typical SAW device has a pair of IDTs on a piezoelectric body to utilize SAWs. AC power fed into the input IDT is converted into mechanical energy at the surface of the piezoelectric body. Since the IDT has a comb-like shape, dense portions and coarse portions are produced in the piezoelectric body, resulting in an acoustic wave. The acoustic wave propagates along the surface of the piezoelectric body to reach the output IDT. Then, the SAW is converted again into electrical energy by the output IDT to be used as the output.
The types of piezoelectric materials used for the above purpose include bulk single crystals, such as LiNbO3 and LiTaO3, and a thin ZnO film that is vapor-grown on a substrate. Commonly used types of materials at the present include a single-crystalline piezoelectric body, a ZnO piezoelectric body grown on glass, and a ZnO piezoelectric body grown on sapphire. An increase in the amount of information transmission in recent years has caused the transmission signal to extend into the microwave range, so there is a growing demand for devices that can be used in the 10-GHz band.
Generally, the operation frequency of a SAW device is determined by the propagation velocity and wavelength of the SAW. The wavelength is determined by the distance across one cycle of the lines of the IDT. When an IDT having the same distance across one cycle of the lines is used, i.e., when the same wavelength is used for operating a SAW device, the SAW device having a higher wave propagation velocity in the piezoelectric material can be used at a higher frequency.
The single-crystalline piezoelectric body LiNbO3 yields a propagation velocity, V, of 3,500 to 4,000 m/s, and LiTaO3, of 3,300 to 3,400 m/s. A ZnO piezoelectric crystal grown on a glass substrate yields about 3,000 m/s at maximum. The conventional materials having such a low velocity cannot be used in the 10-GHz band unless the line width is decreased to less than 0.5 xcexcm. To solve this problem, a method in which diamond, which has the highest sound velocity among the materials (transversal wave velocity: 13,000 m/s, longitudinal wave velocity: 16,000 m/s), is used as a substrate has been devised (see, for example, published Japanese patent application Tokukaishou 64-62911).
However, even though a propagation velocity of 10,000 m/s is achieved by the use of diamond, it is necessary to provide by fine processing IDTs having a line width of 0.25 xcexcm (the line width is equal to a quarter of the wavelength) to produce a SAW device that operates in the vicinity of a central frequency of 10 GHz. Such IDTs cannot be mass-produced under the current conditions of the process technology. Generally, a SAW device operates with higher efficiency when it has a larger electromechanical coupling coefficient. Here, the coefficient represents a measure of the conversion efficiency when electrical energy is converted into mechanical energy. In particular, it is desirable that the electromechanical coupling coefficient be 0.5% or more.
In the case of a thin piezoelectric film formed on a substrate, the propagation velocity and electromechanical coupling coefficient depend not only on the materials of the piezoelectric film and substrate but also on the thickness of the piezoelectric film and the line width of the IDTs. It is necessary to utilize a harmonic of a SAW for producing a SAW device that operates in the vicinity of 10 GHz with a mass-producible line width of 0.5 xcexcm or more. An object of the present invention is to offer a SAW device that operates in the vicinity of 10 GHz with high efficiency. The present invention achieves this object by utilizing a harmonic of a SAW that allows the IDTs to be provided with a mass-producible line width of 0.5 xcexcm or more.
In the present invention, the thickness of a ZnO film is expressed in the following formula:
(2xcfx80xc2x7H/xcexM),
where H: the actual thickness of the ZnO film; and
xcexM: the wavelength of a harmonic of a SAW that propagates along the film.
Similarly, the thickness of a diamond substrate is expressed in the following formula:
(2xcfx80xc2x7HD/xcexM),
where HD: the actual thickness of the diamond substrate.
These formulas are parameters with no dimension.
Experiments carried out by the present inventors reveal that the film thicknesses H and HD affect the propagation velocity V and the electromechanical coupling coefficient K2 by the ratios to the wavelength. It is therefore, useful to employ the foregoing parameters in classifying the conditions. FIGS. 1 to 4 show the cross-sectional views of the SAW devices of the present invention. These devices are called simply the I, II, III, and IV types.